Method for manufacturing a dental restoration

ABSTRACT

A method for manufacturing a dental restoration, including: determining an external form and dimensions available for a completed restoration; obtaining an image of a natural tooth to be replaced with the restoration or a tooth corresponding therewith, wherein the image comprises at least the external surface visible in use of the to be replaced or corresponding tooth, with variations in the appearance therein; defining locally on and at least to visible depth below the surface of appearance-determining properties of at least one material to be applied for the restoration in accordance with the obtained image and the variations in the appearance therein; constructing the restoration, including the steps of: providing at least one material to be applied in non-cohesive form; and providing cohesion to the material in accordance with the available form and dimensions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a dentalrestoration.

2. Description of the Related Art

Different methods are known in the art for manufacturing a dentalrestoration, wherein these methods are mainly determined by the type ofmaterials used.

Ceramic materials have been successfully applied for many years indental restorations. The two most important functions of restorativeceramics are aesthetics and strength. However, most dental ceramics donot fulfil both functions. Different porcelains and glass ceramics areused for aesthetic applications. These have a natural tooth colour andtranslucence but have a relatively low three-point bending strength(50-200 MPa).

Higher-strength materials have been developed as basic core material.These materials have a three-point bending strength of 150 to 500 MPa,but are generally too opaque and must be combined with translucentporcelain veneered thereon. The fracture toughness however remains lowcompared to metal alloys, whereby these ceramics are susceptible toproduction errors and stress concentrations, such as occur when there isa less than optimal fit between prepared tooth and the restorativeceramic.

Medically pure yttrium-stabilized tetragonal zirconium oxide (Y-TZP) ischaracterized by both aesthetics and a high three-point bendingstrength, higher than 1000 MPa, with a great toughness and an excellentresistance to slow crack development. Zirconium oxide has heretoforebeen applied very successfully in orthopaedics as hip-joint ball. It hasalso been very successfully introduced in dentistry for root pins. Ithas further been found to have a very low susceptibility to dentalplaque. However, zirconium oxide has a white colour not suitable fordental restorations and must be coloured with a colour correspondingwith the tooth colours.

In DE 4207179 A1 (1992) Yoshida describes a method of colouringorthodontic zirconium oxide components by adding a mixture of erbium-,praseodymium-, iron- and zinc oxides. Some additives, such as zinc,result in serious degradation of the physical properties of the sinteredzirconium oxide. In DE 19938143 A1 erbium-, iron- and manganese oxideare mainly applied as colouring oxides.

Effect of suitable colour additives to sintered Y-TZP zirconium oxide

Colour additive Form Effective colour Note Iron Fe₂O₃ BrownConcentration lower than 1% by weight Erbium Er₂O₃ Light violet Formssolid solution with ZrO₂ Praseodymium Pr₂O₃ Deep yellow Forms solidsolution with ZrO₂

These are first dissolved in hydrochloric acid and added to zirconiumoxide and admixtures dissolved in hydrochloric acid. By hydrolysis withammonia, annealing of the deposition and fine-grinding, an homogeneouslycoloured zirconium oxide powder is obtained which can be furtherprocessed. This takes place by mixing the powder with binders (forinstance 2% by weight polyvinyl alcohol and 0.15% by weight oleic acid),and making this into a geometrical shape by pressing. The binder isburnt out for 0.5 to 2 hours at a temperature of 850-1000° C. A dentalrestoration, enlarged by the sintering shrinkage factor, is then cut outby means of a CAD/CAM system and the product is sintered to increasedensity at 1300-1500° C. for 2-4 hours. The result is a monochromecoloured restoration.

It is however desired in dentistry to colour the restoration locally.The base structure of a crown, in the form of a cap, is thus colouredslightly browner at the bottom, the so-called shoulder portion, andlighter yellow in the sections in the cutting edge area.

At the moment the restorations are further produced in automatic mannerwith fairly traditional cutting technique, with proportionately highmaterial loss. With the use of zirconium oxide the price of the materialis considerable and a net-shape production method would be desirable.Accurate colouring with locally occurring variations is complex here, ifnot impossible.

Up to the present there has been no possibility of making restorationsusing rapid prototyping or even in automated manner which have locallydifferent properties or colours. In addition to the absence of anexperimental configuration, there has also been no possibility oflocally colouring product files.

In order to obviate or at least reduce the above stated and otherproblems and drawbacks of the known art, a new method for manufacturinga dental restoration is provided according to the present invention.

SUMMARY OF THE INVENTION

With a method according to the invention it is possible to manufacture arestoration with a very natural appearance, in respect of both colour,translucence and so on and the shape thereof.

A possible embodiment of a method according to the invention can bedesignated as 3D printing. Experimental configurations have recentlybeen constructed, particularly at the Massachusetts Institute ofTechnology (MIT, Boston, USA), with which it is possible using softwareto determine the colours of the STL files (input files for rapidprototyping machines). Owing to the development of new materials andmachines, the possibilities in the field of 3D printing have recentlybeen greatly expanded, although there is still no application known forusing this technology for dental restorations. The printer is used toapply an organic binder to a powder compacted using a roller, wherebycomplex shapes can be produced.

The principle of 3D printing (3DP) is known. The method was developed bythe institute of technology of Boston, MIT. Market developments of theprinciple have since taken place. Zcorporation for instance has howeverused starch, which has little environmental impact and is freelyavailable, as binder.

A drawback to the use of starch as binder is that it increases theshrinkage of the bound ceramic powder and that the shrinkage canmoreover not be predicted in all directions. This makes starch lessuseful for the accuracy desired in dentistry. This in contrast to coldisostatic pressed (more than 2000 bar) ceramic, which after cutting andsintering results in a sufficiently accurate restoration. A newmaterial, ZP100, was developed as an alternative, with which thin-walledand complex products such as dental restorations can indeed be printedin 3D with sufficient accuracy.

It is furthermore possible to “build in” a natural colour layer by layerby adding Pr-oxide (yellow), Fe-oxide (brown) and Er-oxide (violet) aspigment to the binder. The quantities to be applied amount for each ofthese colour additives to between for instance 0.05% and 2.0% of theweight.

In a preferred embodiment a method according to the invention, byproviding the material layer by layer, the depth to which this materialmust be processed to provide it with cohesion remains limited. Theproduction process hereby remains readily manageable and leads to goodresults. Particularly good results are achieved in respect of theappearance of the restoration to be produced (colour, translucence andso on), if certain measures are herein also applied. Colour variationscan thus be realized in a three-dimensional direction over the outersurface of the restoration to be produced, while colour variations couldonly be realized in the direction in which the layers are successivelyprovided on each other when layers to be provided separately have ahomogeneous composition. A variation resulting in colour variation canalso be realized in the layers, so that even more natural variations canbe obtained.

In yet another preferred embodiment the non-cohesive form of thematerial to be applied can be one of powder form, liquid, thin slicesand so on. This is related to the method to be applied for providingcohesion therein. In a powder form the cohesion can for instance begiven by selectively adding binder to the powder or by subjecting thepowder to the action of at least one laser. Particularly in the case ofliquid forms of the material to be applied, an arrangement with twolasers can be used depending on the capacity of this liquid material totransmit laser light, where the intended effect of providing cohesion,for instance solidifying, only occurs where the laser beams coming fromthe lasers cross each other. In an embodiment with thin slices colourvariations can be realized per slice around the periphery thereof, orthe slices can be manufactured from homogenous material.

The material to be applied in the method is for instance ceramicmaterial, porcelain, glass ceramic, and so on, but preferablyyttrium-stabilized tetragonal zirconium oxide, to which the invention ishowever not limited. This has properties which are very advantageous inrespect of the invention.

In order to allow variation in the appearance over the outer surface ofthe restoration to be manufactured, for instance in respect of thecolour thereof, many materials can be used, such as erbium oxide, ironoxide and praseodymium oxide, manganese oxide and so on. Such materialscan be combined well with the materials to be applied in the restorationto be manufactured, particularly zirconium oxide.

The present invention will be elucidated on the basis of an embodimenthereinbelow and a description of the one annexed drawing, which showsschematically a partly cut-away perspective view of an embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a possible embodiment of an installation asimplementation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Installation 1 comprises a holder (not shown) in which layers ofpowder-form yttrium-stabilized tetragonal zirconium oxide (Y-TZP) can beplaced one over another. The zirconium oxide is in powder form so that abody 2 of this powder is formed as the stacking of said layers onto eachother progresses.

The layers are arranged using a powder holder 3, which in thisembodiment is funnel-shaped and adapted to pour a measured quantity.Powder holder 3 is provided for this purpose with a flap 4 with whichthe pouring opening of powder holder 3 can be closed.

If a quantity of powder-form zirconium oxide from powder holder 3 ispoured, either at one location or spread over the upper surface of thebody 2 formed so far, the upper surface is leveled using a driven roller5 which is coupled to a motor 6. The powder-form zirconium oxide is notonly leveled, but also compressed and compacted to the desired extent.

The installation further comprises a laser generator 7 connected to acontrol 8. The focal point 9 of the laser generator acts on the uppersurface of body 2 to give the powder-form zirconium oxide cohesion inthe upper layer, which has just been poured from powder holder 3 andleveled and compacted by roller 5. Under the influence of the action ofthe focal point of laser generator 7 there also occurs adhesion to thematerial lying beneath the last poured layer. Any random shape can thusbe generated in a layered structure, precisely this being advantageousin the case of dental restorations because of the usually erratic shapesthereof.

The FIGURE shows such a dental restoration 10. The method formanufacture hereof is being performed, so that the dental restoration 10is only partly shown. A part of the body 2 of loose powder is furthercut away in the view in order to show the dental restoration 10.

The focal point 9 of laser generator 7 follows a pattern in the materialof the last spread layer such that, after processing a number of layersin this manner, a desired three-dimensional form of the dentalrestoration can be obtained.

In the areas 11 and 12 of the dental restoration 10 material is added tothe powder-form zirconium oxide prior to the action of focal point 9.This may be for instance iron oxide, erbium oxide or praseodymium oxide.These materials have an effect on the colour, wherein iron oxideproduces a brown discolouration and erbium oxide a light violetdiscolouration, while praseodymium oxide has a deep yellow colouringeffect.

It is shown clearly in the FIGURE that, round the periphery on the upperside of the dental restoration 10 formed so far in the FIGURE,variations are arranged in the depth in internal direction of layers 11and 12, which can otherwise also be doped with colouring agentsdiffering from those stated above. Colour variation is thus possible inall directions over the outer surface of the dental restoration 10 to beformed. The properties of the material for doping in areas 11 and 12 canbe varied from the bottom to the top, just as around the surface shownin the FIGURE of a cross-section of a dental restoration 10 to beformed.

The installation 1 shown in the FIGURE is also referred to as a colourprinter.

As an alternative within the scope of the present invention to the lasergenerator 7 shown in the FIGURE, it is also possible to work with abinder which gives the non-cohesive powder-form zirconium oxide cohesionlocally and where desired. As shown in the FIGURE, this can also takeplace in a structure with layers. The binder is active up to a depth towhich the binder is able to give cohesion to the powder-form zirconiumoxide. In such an application this active depth is of course greaterthan the thickness of a layer of powder-form zirconium oxide which hasbeen applied or is to be applied, so that the adhesion to underlyingparts of the restoration 10 to be formed is also brought about.

Dental restorations are manufactured with colour printer 1 from a loose,dry zirconium oxide powder with 3% yttrium oxide. The geometry for therestorations originates from a scan-design system such as a CAD-CAMsystem. Printer 1 is used to apply an organic binder to powder compactedwith the roller, whereby complex shapes can be produced. A solution of4% by weight 4AC (Hercules, USA) is made as binder. Three printsuspensions are made herefrom by adding respectively 0.05% Pr-oxide,0.05% Fe-oxide and 0.05% Er-oxide. The pigment is added to the binder asmicronized powder and mixed in a turbine agitator. In a colour printer abridge is imprinted with an STL data file, with differentiatedcolouring, obtained from a CAD system for dental restorations. Afterconstruction the bridge is heated to 650° C. at 5° C./min. Therestoration is then heated to 1500° C. at 10° C./min and held there for2 hours. The bridge exhibits the designed local differentiatedcolouring.

1. A method for manufacturing a dental restoration, comprising the stepsof: determining an external form and dimensions available for acompleted restoration; obtaining an image of a natural tooth to bereplaced with the restoration or a tooth corresponding therewith,wherein the image comprises at least the external surface visible in useof the to be replaced or corresponding tooth, with variations in theappearance therein; defining locally on and at least to a visible depthbelow the surface of appearance-determining properties of at least onematerial to be applied for the restoration in accordance with theobtained image and the variations in the appearance therein; andconstructing the restoration, including the steps of: repeatedlyproviding layers of at least one material to be applied in non-cohesiveform; and for each layer providing cohesion to the layer of at least onematerial with previous layers in accordance with the available form anddimensions, where the step of providing at least one material to beapplied includes: varying the at least one material to be applied inaccordance with variations in the image in a plane defined by each ofthe layers around the periphery of the surface of a layer cross sectionof the dental restoration to be formed, wherein the at least onematerial is in powder form including an additive configured to bring theappearance of the restoration to be formed into accordance with theimage, the additive comprising at least one of the following: erbiumoxide (Er₂O₃), iron oxide (Fe₂O₃), praseodymium oxide (Pr₂O₃) andmanganese oxide (Mn₂O₃), wherein the step of applying the cohesion isperformed by sintering of the powder without the use of a bindingadditive and by an action of a laser.
 2. The method as claimed in claim1, further comprising the steps of providing layer by layer innon-cohesive form at least one material to be applied; and providingcohesion to the material in each of the layers in accordance with theavailable form and dimensions as well as providing adhesion to apreceding layer before providing a subsequent layer.
 3. The method asclaimed in claim 1, including providing as the material to be applied atleast one of ceramic material, porcelain, glass ceramic andyttrium-stabilized tetragonal zirconium oxide (Y-TZP).
 4. The method asclaimed in claim 2, further comprising the step of leveling a providedlayer of material, prior to providing the material with cohesion.
 5. Themethod as claimed in claim 1, wherein the determining step is performedusing a CAD-CAM system.